In recent years, with the rapid development of mobile communication systems the demand for small-sized and high performance mobile communication equipment is going up more and more. In order to satisfy such a demand, the applicant of the application concerned proposed earlier a thin film multi-layer electrode of thin film conductor layers and thin film dielectric layers having fixed thickness which are alternately laminated, to realize a low-loss electrode.
For example, in a circular TM mode resonator, a thin film multi-layer electrode formed in a method to be described hereinafter has been used.
That is, as shown in FIG. 6, the circular TM mode resonator 53 with open-ended side comprises a thin film multi-layer electrode 52 of layers of thin film conductor and dielectric substance alternately formed by sputtering and using a metal mask on the main surface of a circular dielectric substrate 51 both the main surfaces of which have been ground to be flat. Further, although not illustrated in FIG. 6, a thin film multi-layer electrode is formed on the lower side of the circular dielectric substrate 51 as on the upper side. FIG. 7 is an expanded sectional view in the vicinity of the external portion of the resonator 53. A thin film multi-layer electrode 52 is formed in such a way that as shown in FIG. 7, a couple of thin film conductor layers 54 and thin film dielectric layers 55 are alternately given on the dielectric substrate 51. In the vicinity of the external portion (the righthand side of FIG. 7), the thin film conductor layers 54 and thin film dielectric layers 55 are in a tapered shape. This is because sputtered particles migrate into a very little gap between the metal mask and dielectric substrate 51 when the thin films are formed by sputtering. Further, in the external portion 56 of the dielectric substrate 51 the thin film multi-layer electrode 52 is not formed because the external portion is pressed and covered by a metal mask fixed to the dielectric substrate in the formation of thin films by sputtering. Line X--X in FIG. 7 shows a masking line defined by the metal mask.
However, the above-mentioned conventional circular TM mode resonator 53 has had a problem to be described hereinafter.
First, regarding the thin film multi-layer electrode 52 to be formed on both the main surfaces of the dielectric substrate 51, it is difficult to form the thin film multi-layer electrode formed on one main surface and the thin film multi-layer electrode formed on the other main surface so that both of the electrodes lie exactly one on top of another, on opposite surfaces of the dielectric substrate 51. That is, there are cases in which the electrodes are displaced from each other.
Further, in the conventional circular TM mode resonator 53, because the external portion 56 of the dielectric substrate 51 remains as an excessive dielectric material, the stray capacitance between the thin film multi-layer electrodes formed on both the main surfaces has become large.
More, although the thin film conductor layers 54 should be electrically insulated from each other, there is a chance of an electrical short-circuit at the tapered part of the external portion of the thin film multi-layer electrode 52.
The three things pointed out in the above have caused the conventional thin film multi-layer electrode to be deviated from a boundary condition for its original low-loss operation. For example, in an open-ended circular TM mode resonator 53, the conductor loss inside the resonator is increased and no-load Q of the resonator is degraded.
Further, although the resonance frequency of the open-ended circular TM mode resonator 53 is determined by the diameter of the circular thin film multi-layer electrode 52, when the thin film multi-layer electrode 52 is formed by using a metal mask, as described above, and because the diameter of the thin film multi-layer electrode becomes larger than the diameter of the metal mask, for example, due to sputtered particles migrated between the metal mask and the dielectric substrate 51, it is difficult to form an electrode 52 having a desired diameter.